TORONTO -- Healthcare workers should get first crack at a vaccine against the pandemic H1N1 influenza, the World Health Organization said today.

TORONTO, July 13 -- Healthcare workers should get first crack at a vaccine against the pandemic H1N1 influenza, the World Health Organization said today.

The goal would be to "maintain a functional health system as the pandemic evolves," according to Marie-Paule Kieny, PhD, the director of WHO's Initiative for Vaccine Research.

It will be important to protect healthcare workers so they can care for victims of the pandemic strain, Dr. Kieny said. Also, she said, "people will continue to be ill with other diseases and those diseases will need to be taken care of."

Dr. Kieny said a committee of experts made the recommendation late last week and it was accepted by WHO director general Margaret Chan, MD.

The committee -- the Strategic Advisory Group of Experts, or SAGE --- also said individual countries should target other groups depending on local priorities.

For instance, she said, a country whose goal is to lessen transmission might target children, who are "amplifiers of infection" because they are often in school or care groups.

The committee said pregnant women and adults ages 15 through 49 are other groups that might have priority access to the vaccine when it is available.

Dr. Kieny said more than 90% of next fall's seasonal vaccine is now complete, so there will be no need to switch production in order to make an H1N1 vaccine.

It now seems likely, she said, that an H1N1 vaccine will be available in September and October.

She added that healthcare authorities should prepare for the fall's seasonal flu as if there were no pandemic. "We don't envisage any changes in the recommendations for seasonal vaccination," she said.

Dr. Kieny said the first doses of vaccines for testing have been produced, using a cell-based manufacturing system, and drugmakers say the first doses from an egg-based system should be available soon.

An emerging issue, Dr. Kieny said, is that vaccine yields so far are only "moderately effective."

She noted that different vaccine seed stocks often have varying yields, but the seed stocks created for the H1N1 are giving only about 20% to 55% of what would be seen from what she called "good yielders" in previous years.

"This is not to say that the strains grow poorly," she said, but for some reason the hemagglutinin antigen they produce is not stable.

"It is well known that some strains are good yielders and some are bad yielders," she said. "It happens that for the first series of strains that were generated, we didn't come up with a good yielder."

The WHO vaccine network is now trying to come up with strains that will have better yields, she said.

The current strains are still giving enough vaccine for clinical testing, she said, and there is no reason to be "inordinately anxious" about the low yields.

Clinical testing of the current vaccine strains will not need to be redone if new, better-yielding strains eventually are used, Dr. Kieny said, because the difference will be in yield, not in immunogenicity.

And as WHO prepares its attack plan for the anticipated increase in H1N1, a new concern is emerging -- obese patients may be at increased risk for the pandemic flu.

So suggests a report from the Michigan Department of Community Health that detailed the clinical characteristics of 10 hospitalized H1N1 patients. Nine of the 10 were obese, according to findings published in the July 10 Morbidity and Mortality Weekly Report.

Half of the patients had pulmonary emboli and three of the 10 patients died.

The report detailed only a small number of severely ill patients -- all had H1N1 infection and acute respiratory distress syndrome (ARDS) and all required mechanical ventilation in a tertiary-care ICU -- but the finding was nonetheless striking, the MMWR editors wrote in an accompanying note.

"Whether obesity is an independent risk factor for severe complications of novel influenza A (H1N1) virus infection is unknown," the editors wrote. "Obesity has not been identified previously as a risk factor for severe complications of seasonal influenza."

But the editors pointed out that, in a mouse model, obese mice had "significantly higher mortality when infected with seasonal influenza virus compared with their leaner counterparts."